CMake: switch to GNUInstallDirs (#1424)

CMake: switch to GNUInstallDirs  (#1424)

* Add GNUInstallDirs for standard installation directories

Distributions are given standard variables for already existing hooks.
Multiarch libdirs is taken care of automagically.
Raises minimum cmake version by a little.

* Handle CMAKE_INSTALL_xxx being absolute paths for .pc file generation

In some cases the CMAKE_INSTAL_{BIN,MAN,DOC,LIB,INCLUDE}DIR variables
may turn out to be absolute paths in which case prepending ${prefix} in
the pkg-config .pc files will result in incorrect values.

For .pc file generation, figure out if these variables are absolute and
omit the prefix in the configured file when so.

See: https://github.com/OSGeo/PROJ/commit/ab25e4b7ed9544e668282dcd293cfaaa2e56dbdf